This invention relates to electrical power substations, and more specifically relates to a novel high speed fault diverter switch for such substations which diverts fault current to ground in the event of a failure or arcing somewhere within the substation.
Gas-insulated substations which occupy a small area are well known for the distribution of electrical power at extremely high voltage, for example 345,000 volts. Thus, electrical power substations, which deal with extremely high voltages, have occupied extremely large ground volumes in the past where the equipment used in the substation was air-insulated equipment. Gas-insulated substations utilize the exceptional dielectric properties of electronegative gases, such as sulfur hexafluoride and enclose the substation conductors and apparatus in sulfur hexafluoride, thereby obtaining extreme savings in space since the dielectric gas permits the designer to locate conductors and equipment much closer than when air-insulation is used.
In gas-insulated systems, it is possible that an internal flashover can occur in one of the bus connectors or in one of the components of the substation. Such internal flashover can result in a significant rise in gas pressure within the equipment and in burn-through of the metallic grounded enclosure of the equipment. Both burn-through and gas pressure rise depend upon the magnitude of the fault current and the fault duration.
Pressure rise is affected by the gas volume which, in turn, may relate to the distance between insulators which support the bus conductors within their respective grounded housing and to the support insulator construction -- whether or not the insulators are gas barriers or have gas flow openings therethrough. Even if the insulators have openings, they still would tend to restrict the flow of gas between adjacent sections in the event of a rapid pressure increase due to an internal fault, thereby further permitting the rise in internal pressure in the section containing the fault. The rise in pressure within the substation and its components is in any event extremely rapid and can cause the mechanical failure of the system components, and further causes the distribution of contaminated arcing products throughout the system.
Present protection systems all rely on back-up cicruit breakers which require several cycles to operate in order to clear such faults, and further rely on component design to limit the deleterious effects of a fault within the system.
The burn-through problem described above is extremely serious and tests have shown that a conventional aluminum housing can burn through in about six cycles of 50 kA arcing. Clearly, the burn-through time will vary with enclosure material and thickness. In any event, burn-through arcing will tend to cause weakened regions within the grounded enclosure, and the arcing itself may cause extensive damage to the insulation system.
As pointed out previously, internal arcing in a gas-insulated substation has been dealt with simply by relying upon existing back-up circuit breaker protection and by designing the substation components to limit the degree of damage due to arcing and to limit the spread of contamination products.